This invention relates to a method for heating a fiber reinforced resin material by magnetic induction and, more particularly, it relates to a method for selectively heating a resin composite reinforced with an electrically conductive fiber by magnetic induction without substantially heating the electrically conductive fiber.
U.S. Pat. No 4,871,412 discloses a method of bonding a thermoplastic layer to a substrate, the thermoplastic layer contains electrical conductive fibers such as carbon fibers which are heated by inducing electric currents into the fibers at frequencies in the frequency range of 3-4 MHz. At the frequency range involved, it is believed by the patentee that sufficient voltage is induced in the carbon fiber to cause breakdown between fibers, between adjacent layers and possibly within one layer such that circulating eddy currents heat the carbon fibers which brings the thermoplastic up to a softening or fusion temperature.
U.S. Pat. No. 2,393,541 discloses that by properly selecting finely divided metal particles and alloys having ferromagnetic properties, the heating temperature in the presence of a high frequency magnetic field may be readily limited and controlled to the particular temperature or temperature range necessary for the heat-treating of materials such as glue, adhesives or plastics. When any of the various metals or alloys of the ferromagnetic class become heated to a particular temperature, known as the Curie point therefor, then the ferromagnetic qualities cease. As a result, any further application of a high frequency field is substantially ineffective to cause further heating if the particles are small. Masses of metal of substantial size may be heated by magnetic induction due to the setting up of eddy currents, as well as because of hysteresis effects in the case of metals of the ferromagnetic class. The patentee discloses that since the ferromagnetic particles are finely divided and effectively insulated from each other by the adhesive or other dielectric material mixed therewith, there is no substantial heating above the Curie point due to eddy currents and that particles may be chosen such that the heating effect is discontinued upon reaching the Curie point for the particles so that the non-conductive material is protected against damage if it is of the nature of damage that might occur by more prolonged heating without a further temperature rise.